That will surely be the case for Moltex, that is why I used the sodium reactor analogy.

However I was unaware of the middle term zirconium activation. Maybe thez can handle it by using sufficiently thick shielded casks for material replacement. Or just wait sufficiently long like they do in the other molten salt concepts (if it is possible to do it with all the fission products vented in the primary circuit, I agree that venting here makes material replacement very difficult. Maybe they want to replace all the entire unit at once.)

- venting Xe137, Kr89, etc. is not "harmless". This is a lie. Cesium and Iodine bind to salts, but the Xe and Kr parents don't, and some of these will leave the salt so won't be converted to stable halides. Xe137->Cs137. Kr89->Rb89->Sr89. These are nasty boys that make cyanide look like chocolate. This is well known. The MSRE had to have months of holdup and charcoal beds before they could release these gasses, and that was in time with much less stringent regulations. If Moltex vents constantly, they had better have a holdup system for the gasses or they will severely contaminate their entire containment cell with radiostrontium, radiocesium, and other nasties. It is entirely possible to deal with this by sound and simple engineering, but claiming that simple venting is "harmless" is crazy sales talk.

I had assumed that they would have hold up tanks - as ThorCon and maybe Terrestrial do. It's not explicitly mentioned, but it's so obvious you can't vent these gases to the atmosphere .....

I know they've made sure that the steam tubes have no welds at the points of contact. I suspect that is not enough for the fast design and that will need an intermediate loop.

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- what to do upon cladding leakage? It makes a nasty tub of salt. How is this cleaned or otherwise recoverable? Millions of tiny thin walled tubes with tiny thin walled welds can spring small leaks, this is the experience with nuclear energy.

No welds needed at points of contact. Nevertheless, at some point there will be a leak.....

It looks to me like they could do with an intermediate loop. They certainly have the space to do so and could use large amounts of intermediate salt at slow speed. That could also contribute towards passive cooling and potentially thermal mass to allow partial load following.

I had assumed that they would have hold up tanks - as ThorCon and maybe Terrestrial do. It's not explicitly mentioned, but it's so obvious you can't vent these gases to the atmosphere ....

It is obvious, which is why its such a red flag. The video clearly shows venting to containment. This must be so, as the fuel shuffler wouldn't work if there were vent lines connects. The fuel shuffler clearly would be in the vent path, so would become badly contaminated with Cs137 from Xe137, and similar 'bad bros' such as Kr-Rb-Sr sequence. The video further claims the gasses are "harmless" which is silly and blatantly obviously wrong, there simply isn't enough delay volume in those tubes, nor enough delay time for the 'bad bros' to decay in the tubes with high efficiency. So another red flag there. I know this is a sales video, where a little simplification might be ok, but outright lies are not ok.

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I know they've made sure that the steam tubes have no welds at the points of contact. I suspect that is not enough for the fast design and that will need an intermediate loop.

The only way to do this is to have U-tube type bends in the tubes. Such bends can suffer from thinning. General corrosion can also cause thinning and leaks. Just because a tube is seamless, doesn't mean it can leak. Leaks can occur in places other than welds if there is sufficient surface area and the metal thickness is small enough. Its a tube, a tiny thin tube. It can leak. The driving force is certainly there - hot fluoride salt on one side, hot steam and water on the other, with a phase transition in it to boot. Arguing thousands of tiny thin walled tubes in a boiler with molten salt on the heated side, won't leak, is quite a bad strategy to take with the regulator - you'd have to prove that tens of thousands of tubes won't leak in say 1:million or so, to be outside design basis accident space. So basically single tube leak probability in the billions of operating years. Which would go against all the experience with nuclear steam generators.

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No welds needed at points of contact.

Again, that would need U-tubes or similar arrangement. Seamless molybdenum U-tubes. I'm not aware of this being a proven technology, but Moltex claims that no new technology is needed. Using a nuclear cladding material that no reactor has ever used before doesn't qualify as no new technology in my book, but perhaps the Moltex people are adventurous and optimistic sorts. I've met them personally and I would say they are quite optimistic. It's very refreshing and positive, let me say that at least.

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looks to me like they could do with an intermediate loop.

I agree. They should. But they aren't. That's the point here. We are discussing a specific design here, and it doesn't have an intermediate loop. They could also switch to standard MSR ex core cooling and avoid the difficulties of highly irradiated embrittled tubes, but that is not their design.

Like several others, it seems, I was intrigued by the design of the SSR, and being a long term advocate of MSRs, I contacted Moltex with a few questions of my own. So later, I also followed this thread with interest, and as a number of questions were left unanswered, it seemed appropriate to draw the thread to their attention. Rory O’Sullivan of Moltex has now asked me to play the role of messenger, and to pass on some feedback to the points that have been raised. So what follows is from Rory.

“Please find some comments about the Moltex Energy SSR following a dialogue with members of the team:

- There is no steam near the reactor core, there is a secondary coolant loop through the heat exchanger which is the same salt as the primary salt within the tank ZrF4/NaF/KF. A secondary heat exchanger (boiler) creates steam in the turbine hall which is an adjacent building. The pdf on the learn more page of the Moltex site on "reactor design" does specify that there is a secondary loop, though it does not go into detail as to the coolant.

- Gaseous Fission Products. This is dealt with in their paper on line http://moltexenergy.com/learnmore/Moltex_Gaseous.pdf. The summary is that sufficient time has been allowed for the gases to decay within the top of the fuel tube before they release to the argon containment zone. Kr-89 escapes which is radioactive. But harmless is an accurate description, Kr-89 is routinely discharged to atmosphere during conventional fuel reprocessing and that is seen as acceptable by regulators because of the low biological effect inherent with it being a noble gas. The key point is that Xe-137 which is dangerous because it decays to Ce-137 is virtually all contained in the fuel tubes. All this is set out in the relevant pdf.

- The sacrificial zirconium within the fuel tube scavenges any sulphur, prevents chromium dissolution and tellurium embrittlement as it reacts with the Cl salt first.

- The air cooling system is indeed close to the core, this is possible because the coolant salt is a highly effective neutron shield so neutron activation of argon and nitrogen in air is a non issue.

- The point about cladding failure being "inevitable" is just wrong. Modern clad PWR tubes have failure rates approaching zero. That is with high internal pressure, fuel/clad interactions and the water/zirconium corrosion problem. We have no corrosion, no pressure, no solid/tube interaction. Provided the fuel tubes were properly pressure tested and QC'd, it would be an extremely rare event for a tube to leak. A rupture of a fuel tube will contaminate the coolant and will involve an expensive clean up process. This is the same for any reactor. With the SSR there is no safety risk to the public with this accident.

- High Temperatures – the fuel salt peaks at 1200C in isolated areas at the centre of the tube. The fuel salt near the boundaries are similar to the coolant salt temperature of 650C. This means that no material is exposed to temperatures higher than 650C. This is because the coolant keeps the fuel tubes cool at all times. This is the exact same effect in solid fuel elements. Extensive modelling has been to validate this and experiments are ongoing.

- The moderator in the thermal spectrum version is part of the fuel assembly and will be replaced with the fuel assembly.

- Coolant activation. The coolant in the fast spectrum will be activated and can be recycled or disposed of at the end of the plant life. Coolant activation of an Na K Zr F salt was addressed in ORNL/TM-2006/12. In short activation of the coolant does occur but after 10 years of storage, a reasonable time in decommissioning terms, radioactivity falls to levels allowing straightforward disposal or (preferable) recycling. There is a 10^4 neutron screening effect of the coolant for the fast version. This means that the only materials within the high flux zone are the fuel tubes which are obviously replaced once sufficient burnup has been achieved. Design is such that no humans will ever need to go anywhere near the core. All reactors should be designed this way but this is not possible given the complexity of today’s reactors. Activation of the thorium coolant is troublesome as you will have a wide range of fission products in the fuel salt along with fission in the heat exchangers – circa 1% of fission from thorium including some delayed neutrons. This is a far bigger problem for all other MSRs as they have fuel salt going through the heat exchangers."

LWR is the leading reactor design in the world due to its lower upfront cost. It's cost is lower in Russia, China and Korea and most new reactors are being built to these designs.Terrestrial abandoned some features for low cost and is the MSR design attracting some funds.Organic high boiling liquids may be good moderator coolants in two fluid MSR and may have some traction.

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